This hypothesis has many problems itself.
It sounds clever, but how do they know they need to hide?
What is the purpose of killing off other civilizations, when there is no gain from it?
I could may be see conquest. But you are hypothesizing just silly extermination.
Q: how do they know they need to hide?
A: observation
Q: What is the purpose of killing off other civilizations, when there is no gain from it?
A: wrong conclusion. Killing off others leaves more resources for the victor to harvest.
Q:I could may be see conquest.
A: conquest means bringing assets to bear on a particular target in such numbers that everyone else will notice
But you are hypothesizing just silly extermination.
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Does the lion exterminate antelope (for instance) or just stalk and kill the weak who are exposed and cannot flee? One pride will drive off and kill another in competition for territory. Just use the laws of nature that animals live and die by: as above, so below and vice versa
BTW not my hypothesis - even Stephen Hawking warned of this same possibility in answer to Fermi
Just for the sake of pre-emptive pest control maybe...In the "Killing Star" Zebrowski and Pellegrino postulate aliens who really don't care for the possibility of upstart competition and act as soon as it become obvious that humanity is sending out relativistic spacecraft.
It's also possible that the reason might be incomprehensible...because they're aliens after all...